We seek to understand the process of packaging of the bacteriophage lambda chromosome. The major focus of the work concerns the recognition events by which lambda DNA is selected for packaging into preheads. To understand the recognition process, we are studying genetic variants with altered DNA recognition sequences. One variant is lambda cosl, a lambda mutant produced in vitro which has an altered cohesive end site (cos) and hence is unable to package its chromosomes. We have found that some plaque-forming "revertants" of lambda cosl have apparently acquired a cos-like sequence from the bacterial chromosome. Sequencing studies should reveal how much sequence variation can occur in a functional cos site. A second study involves the packaging interactions between lambda and the related phage 21. We find that the cos sites of lambda and 21 are functionally equivalent, nevertheless lambda and 21 do not package each other's chromosomes. This indicates sequences outsite of cos and functionally unrelated to cos are essential for DNA packaging. We engaged in studies to determine the function of these sequences. We continue to search for additional mutations in the DNA sequences involved in DNA recognition.